4. Aquametry for Pharmaceutical analysis .ppt

Aquametry for Pharmaceutical analysis

1. Aquametry
Dr. Sm Faysal Bellah
Department of Pharmacy
State University of Bangladesh

2. Aquametry can be defined as the quantitative
determination of water. The determination of water is one
of the most important and most widely practiced analysis in
pharmaceutical industry.
Aquametry
Quantitative determination of water is important
because many drugs contain water:
● As a solvent. (E.g. H
2O in Syrup, Suspension or Emulsion)
● As absorbed water (E.g. Absorbed H2O by Powder for Suspension)
● As water of crystallization (E.g. Crystals of salts containing H2O)
● As an adulterant (E.g. Excess Water in Digitalis Leaves)
Physical properties of a drug or a raw material are
modified by its water content. Pharmaceutical procedures
of granulation, tablet formation & coating operations are
affected by water content.
Importance of water determination

3. a) Thermal method:
Physical methods of water determination
The technique includes the loss of weight by drying.
Both the BP and USP describe such measurement under the
general term “loss on drying”. The limitation of this method
is that such method also involve losses resulting from other
volatile materials or from decomposition.
These measurement can be made more specific by
limiting the decomposition effects at lower temperature,
that is, drying accomplished at reduced pressure.
Interference by other volatile materials can often be
controlled by a process of measuring the increase in weight
of an absorbent selective for water.

4. Absorption agents used for this purpose are:
• Dehydrite (anhydrous magnesium perchlorate)
• Drierite (calcium sulphate)
• Phosphorous pentaoxide
• Barium oxide
• Calcium chloride
• Anhydrous silica gel
An inert gas is allowed to carry the water lost from a
known quantity of sample to the absorbent whose gain in
weight is then determined.

5. Azeotrope
A mixture of two liquids that boils at constant
composition; i.e. the composition of the vapor is the same
as that of the liquid known as azeotrope.
Chloroform and acetone
Alcohol and water
Water (100
o
C) and toluene (110.6
o
C) form an
azeotrope with bp 84.1
o
C, the azeotrope contains 19.6%
water.

6. Azeotropic distillation method
1. A known weight of sample is placed in a flask with an
organic solvent such as xylene or toluene.

2. The flask containing the sample and the organic solvent
is attached to a condenser and the mixture is heated.
The organic solvent must be
- insoluble with water;
- have a higher boiling point than water;
- be less dense than water; and
- be safe to use.

7. Azeotrope
3. The water in the sample evaporates and moves up into
the condenser where it is cooled and converted back into
liquid water, which then trickles into the graduated tube.
4. When no more water is collected in the graduated tube,
distillation is stopped and the volume of water is read from
the tube.

8. b) Azeotropic distillation method (Dean and Stark trap):
The usual procedure is to add a water immiscible
solvent to the material containing moisture (water) and in
this manner to co-distill any water present.
Recondensation of the vapors results in separation
of water from the immiscible solvent making it available
for volumetric measurement.
The hydrocarbons benzene, toluene and xylene are
the solvents usually used in this determination.

9. These solvents with a specific gravity less than 1 (or
density less than water), have the added advantage of
allowing the water to form a layer at the bottom of the
Dean and Stark trap, where it can be measured directly.
Fig: Dean - Stark apparatus

10. Glass flask
Dean-Stark apparatus
Reflux condenser
Fig: Apparatus for
azeotropic distillation method

11. Azeotropic distillation determination of moisture
have extensive applications because of their, simplicity,
economy, efficiency and accuracy.
The method is specially successful for moisture
determination in bulk materials, such as plant parts and
for medicinal soap solutions.
The main disadvantage to this procedure is that
relatively large samples are required , making the
technique unsuitable for trace amount of water in
expensive pharmaceutical materials.
Applications of Azeotropic method

12. Karl Fischer titration is a widely used analytical
method for quantifying water content in a variety of
products.
The fundamental principle behind it is based on the
Bunsen Reaction between iodine and sulfur dioxide in an
aqueous medium.
In 1935, German Chemist Karl Fischer described a
specific titrimetric method for the determination of water,
remains the most generally applicable procedure.
Karl Fischer titration
Chemical methods of water determination
2H
2O + SO
2 + I
2 H
→
2SO
4 + 2HI

13. Composition of Karl Fischer reagent (USP)
Iodine 125 gm
Anhydrous pyridine 170 ml
Anhydrous methanol 670 ml
Liquid sulfur dioxide 100 ml
Karl Fischer reagent is a mixture of-
In recent years, pyridine, and its objectionable odor, have been
replaced in the Karl Fischer reagent by other amines, particularly
imidazole. These pyridine-free reagents are available commercially for
both volumetric and coulometric Karl Fischer procedures.

14. When prepared it is general practice to increase the stability of
the reagent by adding sulfur dioxide to a stock solution of the other
components the day before actual use.
Numerous side reactions may occur among the constituent
substances. Freshly prepared reagent therefore has a strength about
80% of the theoretical value, but this rapidly falls to about 50% in 1
month and 40% in 3 months.
1 ml of Karl Fischer reagent when freshly prepared will react with
about 5 mg (or 3-6 mg) of water.

15. Chemistry of the reaction
In the presence of water, iodine will be reduced and
sulfur dioxide oxidized in the following manner:
H
2O + I
2 + SO
2 2HI + SO
3
The reversibility of the reaction can be prevented by
using large quantity of pyridine. The concentration of
pyridine is sufficiently large so that I
2
and SO
2
are
complexed with the pyridine as C
5H
5N  I
2 & C
5H
5N  SO
2.
C
5
H
5
N  I
2
+ C
5
H
5
N  SO
2
+ H
2
O + C
5
H
5
N 
2 C
5
H
5
N  HI + C
5
H
5
N  SO
3
------------ (1)
rapid

16. Thus, methanol prevent the further reaction of C
5H
5N  SO
3
with water.
The pyridine sulfur trioxide (pyridinium sulfite) compound, an
inner salt, reacts, in turn, with the methanol present to form the
pyridine salt of methyl sulfate.
C
5H
5N  SO
3 + CH
3OH  C
5H
5N
+
(H)
-
SO
4CH
3
------------- (2)
C
5
H
5
N  SO
3
+ H
2
O C
5
H
5
N
+
(H)
-
SO
4
H
The last reaction is undesirable because it is not specific for
water. It can be prevented by using excess amount of methanol.

17. The primary reaction (1) occurs rapidly and permits the direct
titration of any available water with the reagent.
Karl Fisher determination can be performed by direct titrations
or excess of the reagent can be added and the excess can be back
titrated with a standard water-in-methanol solution.
For a direct titration, methanol is added to dissolve the sample
or to assist in the penetration of an insoluble sample. In back titration
procedure, the reagent alone often serves this purpose.

18. Classification of Karl Fischer Titration


Karl Fischer Titration can be of 2 type –
•1. Volumetric Karl Fischer Titration
•2. Coulometric Karl Fischer Titration
Volumetric Karl Fischer Titration


Volumetric Karl Fischer Titration can be defined as a method of titration
where an exact volume of Karl Fischer Reagent is consumed during the
course of titration from which equivalent amount of Water present in the
sample can be detected; i.e. 1 ml of Karl Fischer Reagent is equivalent to
5 mg (or 3 – 6 mg) water.


End – point in this titration is detected by color change in the solution.

19. Volumetric Karl Fischer Titration can be of 2 type
•1. Volumetric Karl Fischer Direct Titration
•2. Volumetric Karl Fischer Back Titration
Purpose of Performing Volumetric Karl Fischer
Titration –
•Volumetric Karl Fischer Titration is performed when
the sample is not colored.

20. Volumetric Karl Fischer Direct Titration
•In Volumetric Karl Fischer Direct Titration, sample is dissolved in excess
anhydrous methanol and the solution is then filtered to remove impurities.


Then, Karl Fischer Reagent is added to the solution drop by drop with the
help of a burette; thus, forming a pale yellow solution.


When all of the H
2O present in the solution will react with Karl Fischer
Reagent, the color of the solution will suddenly change into Dark Brown from
pale yellow; thus indicating the End – Point of the Titration.

21. Volumetric Karl Fischer Direct Titration

22. Volumetric Karl Fischer Back Titration


In Volumetric Karl Fischer Back Titration, at first the sample is mixed with
excess Karl Fischer Reagent giving the solution a Dark Brown color of
excess Karl Fischer Reagent, since all of the water in the sample has
already reacted with the Karl Fischer Reagent.


Then, A Standard Water – in – Methanol is added drop by drop to the
solution with the help of burette.
•When all of the H2O – in – Methanol reacts with Excess Karl Fischer
Reagent, the color of the solution will suddenly change into Pale Yellow from
Dark Brown; thus indicating the End – Point of the Titration.

24. Purpose of Performing Volumetric Karl Fischer Back
Titration
•Purpose of Performing Volumetric Karl Fischer Back Titration is to
determine the accuracy of the Volumetric Karl Fischer Direct Titration.
•

After the reaction in Volumetric Karl Fischer Direct Titration is complete,
the excess amount of Karl Fischer Reagent is determined by titration with a
standard H2O – in – Methanol Solution.
•

The Actual Amount of Karl Fischer Reagent reacting with desired amount
of Water is calculated by subtracting the volume consumed in the Back
Titration from the volume added in the Direct Titration.

25. Coulometric Karl Fischer Titration
•Coulometric Karl Fischer Titration can be defined as a method of titration
where an exact volume of Karl Fischer Reagent is consumed during the
course of titration from which equivalent amount of Water present in the
sample can be detected; i.e. 1 ml of Karl Fischer Reagent is equivalent to
5 mg (or 3 – 6 mg) water.


End – point in this titration is detected by sudden change in the electricity
current flow.


Coulometric Karl Fischer Titration can be of 2 type –
•1. Coulometric Karl Fischer Direct Titration
•2. Coulometric Karl Fischer Back Titration

26. Instrument
Dry cell
Resistance
The Coulometric Karl Fischer Titration vessel is fitted with 1.5 –
2.9 V Dry cell across a variable resistance of about 2000
which is in series with two platinum electrodes and a
microammeter, mechanical stirrer and a burette

27. Purpose of Performing Coulometric Karl Fischer
Titration –
•Coulometric Karl Fischer Titration is performed when the sample is colored.
•As a result the End – Point cannot be detected by color change.

29. Coulometric Karl Fischer Back Titration

31. End point detection
An end point in a Karl Fischer titration can be observed visually
based on the color change from pale yellowpale yellow to dark brown color of the
excess reagent (volumetric titration).
An amerperometric or electrometric detection of the Karl
Fisher titration end point is employed and found useful specially when
the sample is colored (coulometric titration).
Under these conditions of constant low voltage with the direct
titration procedure, there is a small constant residual flow of current
until the end point is reached, accompanied by a large increase in
current.

32. Thus, when water is titrated with the Karl Fisher reagent there is
a “kick-off” or major deflection in the microammeter to indicate the end
point when last drop of excess Karl Fisher reagent enter the titration
flask.
Conversely, when a back titration is employed, there is a sudden
drop in current or a “dead-stop” end point occurs.

33. Up to the end point in the direct titration of water with Karl
Fisher reagent, there are iodide ions present but no free iodine.
At the potential used, the system is irreversible and the
electrodes are polarized, that is, they have an impressed potential
with a little flow of current.
However, as the free iodine enters the system there is a
reversible iodine–iodide couple established with the depolarization of
electrodes and an increase in the flow of current.

34. Limitation
sThe Karl Fischer reagent is highly specific for water but there
are some limitations-
 Compounds which react with either iodine or iodide
will interfere the process. For example, ascorbic acid will be
oxidized by the iodine present in the reagent.
 The optimal pH range for the Karl Fischer reaction
is from 5 to 8, and highly acidic or basic samples need to be
buffered to bring the overall pH into that range.
 Carbonyl compounds under the conditions of the
Karl Fischer determination react with methanol with the
formation of acetals or ketals and the liberation of water.
R
2
CO + 2CH
3
OH R
2
C(OCH
3
)
2
+ H
2
O

35. Advantage of
analysis
The popularity of the Karl Fischer titration is due in
large part to several practical advantages that it holds
over other methods of moisture determination,
including:
* High accuracy and precision
* Selectivity for water
* Small sample quantities required
* Easy sample preparation
* Short analysis duration
* Nearly unlimited measuring range (1ppm to 100%)

36. Advantage of
analysis
* Suitability for analyzing:
o Solids
o Liquids
o Gases
* Independence of presence of other volatiles
* Suitability for automation
In contrast, loss on drying will detect the loss of any
volatile substance.
Reference: Karl Fischer titration. (n.d.) In Wikipedia, the free encyclopedia online.
Retrieved from http://en.wikipedia.org/wiki/

37. Karl Fischer Titration
What Is Karl Fischer Titration?
Karl Fischer titration is a titration method that uses volumetric or
coulometric titration to determine the quantity of water present in a
given analyte. This method for quantitative chemical analysis was
developed by the German chemist Karl Fischer in the year 1935,
Today, specialized titrators (known as Karl Fischer titrators) are
available to carry out such titrations.
 
Principle of Karl Fischer Titration
 
The principle of Karl Fischer titration is based on the oxidation reaction
between iodine and sulphur dioxide. Water reacts with iodine and
sulphur dioxide to form sulphur trioxide and hydrogen iodide. An
endpoint is reached when all the water is consumed. The chemical
equation for the reaction between sulphur dioxide, iodine, and water
(which is employed during Karl Fischer titration) is provided below.
I2 + SO2 + H2O → 2HI + SO3

38. Karl Fischer Titration Equipment
•Drying tube, sample injection cap, electrode analysis, Drain cook, a
cathode chamber, detection electrode, rotor, anode chamber, KF
reagent.
•Ingredients of KF reagent: Iodine, Buffer (Imidazole), sulphur dioxide,
solvent (methanol).Karl Fischer Titration Procedure
The Karl Fischer titration experiment
can be performed in two different
methods. They are:
Volumetric determination– This
technique is suitable to determine
water content down to 1% of water.
The sample is dissolved in KF
methanol and the iodine is added to
KF Reagent. The endpoint is
detected potentiometrically.
Coulometric determination – The
endpoint is detected in this
experiment electrochemically. Iodine
required for KF reaction is obtained
by anodic oxidation of iodide from
solution.

39. Karl Fischer Titration Applications
It is used in technical products such as plastics, oils, gases.
It is used in pharmaceutical products.
It is used in cosmetic products.
It is used in the industry.
Advantages of
 Karl Fischer Titration
It is fitted for determining water in gases, liquids and solids.
The coulometric titrator helps in detecting free water, dissolved water,
and
 emulsified water.
It is a swift process which demands a minimal amount of sample
preparation.
Extremely accurate method.
Limitations of
 Karl Fischer Titration
It is a destructive technique.
The solvent consumption is high as the manual volumetric titration demands
reloading during each determination.
Coulometric titration is fitted only for samples that contain a small amount of
water.
Coulometric titration takes extremely long periods to determine.